Finished the job today. No joy resolving the root cracking issue. We tried everything from playing with parameters, rooting outside first rather than inside, laying a mig pass first, using strong backs and so on... we even sent a shell out for stress relief, before welding, just to see if it solved the issue. The MTR for the plate was good too. We had that checked out. We ended up just living with the cracks, filling and capping then gouging the hell out of the second side until we found sound metal. Seams were full RT and we got good results. Still annoying though that we never solved the problem.
I had a feeling it was to do with 50/50 joint prep. On thick plate, in the past, I've found that you will get better results with offset bevels, 2/3 1/3 for example.
Anyway, all of your inputs and suggestions were much appreciated.